The present invention generally relates to wireless communication systems and antenna pointing systems and, more particularly, to a wireless communication system including multiple interchangeable platforms for splitting a payload, a closed-loop pointing control method, and a method for providing continuous coverage over a wide geographic area.
Modern satellites and other wireless communication platforms often employ a large number of narrow spot beams providing a beam laydown that forms a cellular coverage of a wide geographic area. Both satellite systems and stratospheric platform systems have size limitations on the communication payload. These limitations make it difficult to package a large multi-reflector antenna subsystem for spot beam area coverage on a single payload.
In a communication system using spot beams, the same frequency needs to be used at the same time at two or more locations. Therefore, the antenna subsystem needs to be able to sufficiently isolate the signals from each other. This is called spatial isolation and spatial re-use. Typically, the distance between re-uses is 2 to square root of 7 cell radii, measured from center to center of two circular cells, which use the same frequency at the same time. Therefore, in general, smaller spot beams allow re-use at a closer distance than larger spot beams. Because the spot beam or cell size is inversely proportional to a diameter of the antenna aperture, achieving a smaller spot beam size requires a larger reflector.
As is well known to antenna designers skilled in the art, cellular spot beam coverage is best implemented using several reflectors, usually 3 or 4. FIG. 1 shows a model 10 for a 4-shaded re-use pattern covering the lower 48 states of the U.S.A. If a desired geographic area 11 is covered with overlapping circular cells (120, 130, 140, and 150) and the cells (120, 130, 140, and 150) have a 4-shaded repeating pattern, as shown in FIG. 1, then a wireless communication platform 17 carries four reflectors 12, 13, 14, and 15 and each reflector 12, 13, 14, and 15 serves one of the different shades. For example, a first reflector 12 serves cell 120, a second reflector 13 serves cell 130, a third reflector 14 serves cell 140, and a fourth reflector 15 serves cell 150. Each reflector has in its focal plane an array of antenna feeds 16, each of which generates one spot in that antenna's shade. The fact that one focal plane serves only one shade allows each feed to be larger than it would be if all four shades had to be served using one reflector. In the 4-shades model 10, as shown in FIG. 1, the feeds 16 can be twice as large in diameter. Larger feeds have the advantage of providing a “flatter” beam for each cell, with less variation in signal strength from the center to the edge of the cell. However, since the communication payload has size limitations, it is difficult or even impossible to package a multi-reflector subsystem that has four large enough reflectors to provide high quality spot beam area coverage on a single payload. Further, each active communication platform is required to have a standby platform to provide backup in case of a failure of the active platform resulting in high costs.
However, mounting the reflectors on different platforms or satellites has the disadvantage that the potential pointing error increases with the number of platforms. If spot beams are provided by multiple platforms for covering one geographic area, it is necessary that the spot beams are pointed accurately relative to each other and relative to the user coverage area. Spot beam antenna patterns are nominally fixed, but they do change as the platform (e.g., a satellite or stratospheric platform) is disturbed, and as the antenna's characteristics change, for example due to sun-induced warping. These changes in the spot beam pattern can be computer simulated, but they are not easily measured in an operational system. Current systems can check performance at pre-selected points, but they do not provide a global picture of the beam patterns.
Prior art satellite systems sometimes employ star trackers to keep the satellite itself accurately pointed. These systems solve part of the problem, but the antenna can contribute pointing errors even if the satellite itself is pointed perfectly. Star trackers are also expensive, and they add mass to the satellite. Further, prior art communication systems may use closed-loop beacon tracking systems. These systems use a transmitter on the ground and one or more receiving beams on the satellite. The closed-loop operates to null signal, which is often the difference between two component signals. When the tracking signal is nulled, the antenna is correctly pointed at the beacon location. Unfortunately, other parts of the antenna pattern may still be incorrect in other directions. Further providing the error zero at the beacon location may not be the best compromise for overall system performance. Beacon tracking systems also add hardware and mass to the communication platform.
Prior art further includes, for example, U.S. Pat. No. 4,630,058 issued to Brown and U.S. Pat. No. 4,599,619 issued to Keigler et al., both utilizing satellite pointing based on ground measurements of the ratios of signal strengths between narrow-angle and wide-angle beams using beacon signals. U.S. Pat. No. 6,150,977 issued to Wilcoxon et al., discloses a method for determining antenna pointing errors of satellite antenna that produces at least one spot beam having corresponding gain pattern and an antenna adjustable relative to the satellite body. Therefore, pointing adjustments are made in the position of the antenna relative to the satellite rather than to the altitude control systems of the satellite. U.S. Pat. No. 6,135,389 issued to Fowell, discloses a method for steering the payload beam of a satellite in a non-geostationary orbit toward an intended service area having known geographical dimensions in order to obtain improved pointing performance with a corresponding reduction in the demand on onboard hardware and software systems. The method comprises the steps of determining a subterranean target point and a direction fixed in the payload beam, calculating the orientation that points the payload beam direction through the subterranean target point, and maintaining this payload beam orientation using an on-board attitude control system. However, these prior art systems for antenna pointing control apply to antennas having an exceedingly large number of spot beams making it impossible to optimize each spot beam separately. Some elements of prior art systems for antenna pointing control could be used to keep the spot beams provided by multiple wireless communication platforms pointed accurately relative to each other, however, antenna pointing control would be difficult, complex, and unreliable.
As can be seen, there is a need for a wireless communication system that uses multiple wireless communication platforms and therefore allows splitting of the payload. Also, there is a need for a wireless communication system, which includes multiple platforms that are interchangeable and that can be backed up by only one standby platform. Moreover, there is a need for a wireless communication system with a split payload that allows the use of larger reflectors producing smaller spot beams to provide high quality spot beam coverage for a wide geographic area.
In addition, there is a need for a closed-loop pointing control method that can be applied to wireless communication systems having multiple platforms. Also, there is a need for a closed-loop pointing control method that allows the antennas and wireless communication platforms of a wireless communication system with multiple platforms to maintain correct pointing, both relative to each other and relative to the user coverage area. Further, there is a need for a closed-loop pointing method that eliminates the need for star trackers or beacon tracking. Moreover, there is a need for a closed-loop pointing method that adds no mass to the satellite or other wireless communication platform beyond the usual antenna positioning mechanisms.